1. Field of the Invention
The present invention relates to a biologically soluble honeycomb structure with high heat resistance used in NOx removal catalysts and the like.
2. Description of Background Art
Honeycomb structures have been widely used as catalyst carriers and the like. Conventionally, ceramic papers and the like made from ceramic fibers such as alumina-silica by paper-milling have been used as materials for honeycomb structures. In recent years, however, honeycomb structures in which ceramic fibers are not used have been demanded in Europe due to suspected carcinogenic properties of ceramic fibers. As honeycomb structures in which materials other than ceramic fibers are used, a honeycomb structure using glass fibers such as E-glass is known.
However, E-glass fibers have low heat resistance of about 500xc2x0 C. at most. For this reason, honeycomb carriers made from E-glass fibers cannot be used as a carrier for NOx removal catalysts which may be used at high temperatures of above 500xc2x0 C.
An object of the present invention is, therefore, to provide a honeycomb carrier with superior biological solubility and high heat resistance.
In view of this situation, the inventor of the present invention has conducted extensive studies and, as a result, has found that a honeycomb carrier with superior biological solubility and high heat resistance can be obtained by forming a honeycomb structure using biologically soluble fiber. This finding has led to the completion of the present invention.
Specifically, the present invention provides a honeycomb structure made from a nonwoven fabric containing biologically soluble fiber.
The honeycomb structure of the present invention is formed from a nonwoven fabric in which a biologically soluble fiber is used. A non-woven fabric obtained by paper-milling a biologically soluble fiber and adding a binder and the like can be given in the present invention.
The biologically soluble fiber in the present invention is defined as a fiber exhibiting a fiber solubility (the amount of fiber dissolved) of 3.5% or more, preferably 5.0% or more, when 1.0 g of such a biologically soluble fiber passing through a 200 mesh sieve is mixed with 150 ml of a physiological saline solution at 40xc2x0 C. and the mixture is shaken by horizontal shaking in a 300 ml conical flask with a turncock at a rate of 120 times/minute for 50 hours. The physiological saline solution used here is a solution prepared by dissolving 6.780 g of sodium chloride, 0.535 g of ammonium chloride, 0.268 g of sodium hydrogencarbonate, 0.166 g of sodium dihydrogencitrate, 0.059 g of sodium citrate dihydrate, 0.450 g of glycine, 0.022 g of calcium chloride, and 0.049 g of sulfuric acid in 1 l of water, and has a pH of 7.4. If the fiber solubility is 3.5% or more after 50 hours, the biologically soluble fiber inhaled by humans is dissolved in the body and loses the shape of fiber. Therefore, there is no possibility that biologically soluble fiber has a harmful influence on the human body.
The following three types of biologically soluble fibers can be given as examples of the fibers exhibiting the above biologically soluble properties. The first type of biologically soluble fiber comprises usually, 60 to 72 wt % of SiO2, 15 to 27 wt % of CaO, 12 to 19 wt % of MgO, and 0 to 13 wt % of TiO2, preferably 63 to 69 wt % of SiO2, 15 to 20 wt % of CaO, 12 to 16 wt % of MgO, and 0.5 to 5 wt % of TiO2.
The second type of biologically soluble fiber comprises SiO2, MgO, and TiO2 as essential components, which form an amorphous substance in the structure. This second type of biologically soluble fiber preferably comprises 60 to 80 wt % of SiO2, 15 to 28 wt % of MgO, and 4 to 20 wt % of TiO2. The second type of biologically soluble fiber may further comprise 0 to 10 wt % of MgO and 0 to 10 wt % of ZrO2.
The third type of biologically soluble fiber comprises SiO2, MgO, and manganese oxides as essential components. Any compound containing oxides of manganese in any form such as MnO and MnO2, for example, may be used as the manganese oxides. Given as the examples of third type of biologically soluble fiber are fibers comprising preferably 60 to 80 wt % of SiO2, 15 to 30 wt % of MgO, and 0.5 to 20 wt % (as MnO) of manganese oxides, more preferably 65 to 80 wt % of SiO2, 15 to 28 wt % of MgO, and 0.2 to 20 wt % (as MnO) of manganese oxides, still more preferably 65 to 80 wt % of SiO2, 15 to 28 wt % of MgO, and 0.4 to 20 wt % (as MnO) of manganese oxides, and particularly preferably 70 to 80 wt % of SiO2, 15 to 28 wt % of MgO, and 0.4 to 20 wt % (as MnO) of manganese oxides. The first to third type of biologically soluble fibers may contain other components inasmuch as the composition of SiO2 and the like is in the above range.
Part of the SiO2, MgO, and the like in the above biologically soluble fibers is dissolved in a physiological saline solution. When CaO is included, part of the CaO is dissolved. The biologically soluble fiber with the above composition not only excels in biological solubility, but also exhibits high heat resistance of at 1,000xc2x0 C. or more.
The biologically soluble fiber used in the present invention has usually an average fiber diameter of 1 to 20 xcexcm, preferably 1 to 4 xcexcm. The average fiber diameter in the above range is preferable because nonwoven fabrics made from the biologically soluble fiber by paper milling have large voids.
The nonwoven fabrics used in the present invention can be obtained by paper-milling of the above biologically soluble fiber and adding a binder. As examples of binders used in the present invention, organic binders such as polyvinyl alcohol and acrylic binder, and inorganic binders such as colloidal silica, and the like can be given. These binders may be used either individually or in combination of two or more.
The nonwoven fabrics may further comprise organic fibers, if necessary. As examples of the organic fibers, pulp, rayon fiber, vinylon fiber, acrylic fiber, and PET fiber can be given. Inclusion of organic fibers in the nonwoven fabrics does not only improve corrugating processability, but also increases the void ratio of the nonwoven fabrics because organic fibers burn and are eliminated by calcination.
When the biologically soluble fibers and binders are made into paper, usually 0.5 to 5 parts by weight, preferably 1 to 3 parts by weight of the binders are used for 100 parts by weight of the biologically soluble fibers. When organic fibers are optionally used, the amount is usually in the range of 5 to 40 parts by weight, preferably 10 to 35 parts by weight, for 100 parts by weight of the biologically soluble fibers. This range of organic fibers is preferable because the resulting nonwoven fabrics have a proper range of void ratio. As the paper-milling method, a conventional method using a round net paper-milling machine and the like can be given, without any specific limitation.
The biologically soluble fibers and binders are dispersed in water during the paper-milling process. The concentration of the biologically soluble fibers in the slurry is usually 0.3 to 2 wt %, preferably 0.5 to 1.5 wt %. The slurry concentration of the above range ensures good dispersion, which results in papers with homogeneous composition and thickness.
The nonwoven fabrics have an inter-fiber void ratio usually of 60 to 95%, preferably of 70 to 90%. If the inter-fiber void ratio is less than 60%, it is difficult to cause the catalyst to be carried by the honeycomb structure. If the inter-fiber void ratio exceeds 95%, the strength of the resulting nonwoven fabrics is insufficient and the amount of catalyst carried by the honeycomb structure easily decreases.
The nonwoven fabrics have usually a thickness of 0.05 to 2.0 mm. If the thickness is less than 0.05 mm or more than 2.0 mm, corrugating the nonwoven fabrics becomes difficult.
The honeycomb structure of the present invention is formed from a plate form nonwoven fabric made from biologically soluble fibers and a waveform nonwoven fabric by alternately laminating and causing the plate form fabric and the waveform fabric to adhere. To form a waveform nonwoven fabric, a flat nonwoven fabric, for example, may be processed using a commonly used corrugating machine.
As the method of causing the alternately laminated flat nonwoven fabrics and waveform nonwoven fabrics to adhere, a method of applying an adhesive to the hills of waveform nonwoven fabric and causing the waveform nonwoven fabric to adhere to the flat nonwoven fabric can be given, for example. As an adhesive, the adhesive containing an organic binder and, if required, an inorganic binder and inorganic filler can be given.
In the honeycomb structure of the present invention, the interval between two juxtaposing hills, in other words, the honeycomb pitch, is usually in the range of 1.0 to 20 mm. The height of the honeycomb cell in the honeycomb structure of the present invention is usually in the range of 0.5 to 10 mm.
The honeycomb structure of the present invention has a high heat resistance and is free from deformation by heat when used at high temperatures. Specifically, the volume shrinkage rate when heated for 3 hours at 800xc2x0 C. is usually less than 10%, preferably less than 6%.
The honeycomb structure of the present invention can be used as a carrier for catalysts and adsorbents. The honeycomb structure of the present invention can be preferably used as a NOx removal carrier for catalysts utilizing the high heat resistance of 800xc2x0 C. or more. As specific examples of NOx removal catalysts, WO3xe2x80x94V2O5xe2x80x94TiO2, V2O5xe2x80x94TiO2, WO3xe2x80x94TiO2, and the like can be given.